Transmitting circuits, with thermionic valves



April 24, 1934. RUSPQLI 1,956,134

TRANSMITTING CIRCUITS WITH THERMIONIC VALVES Filed Sept. 10, 1931J*:T#a:

5, fluspa/ Patented Apr. 24, 1934 UNITED STATES TRANSMITTING CIRCUITS,WITH THERMIONIC VALVES Edmondo Ruspoli, Paris, France ApplicationSeptember 10, 1931, Serial No. 562,167 In Italy March 30, 1931 3 Claims.

This invention has reference to thermionic valve oscillators.

It is well known that oscillations of very constant frequency may begenerated in thermionic valve oscillators in which the piezo-electricaction of a quartz (or similar) crystal creates a condition of the gridcircuit allowing reaction and grid excitation to take place. But it isalso known that the power which may safely be applied to such anoscillator is very low. Beyond this power limit, the increasedmechanical vibrations, voltages and currents cause damage to thecrystal.

Other thermionic valve oscillators are known where a crystal is used tostabilize the frequency, but where a certain degree of reaction and gridexcitation are provided independently of piezoelectric action. If thecrystal is removed, the circuit will oscillate without stabilization, orwill be near the starting point of oscillations.

But here again, if the crystal is to be kept free from danger when it iseffectively stabilizing the frequency, the power limit must be kept low,in the types of oscillators generally used.

The object of the present invention is to greatly reduce the strainusually applied to a quartz crystal in an oscillator, without losing theimportant benefit of the constancy which its use confers to thefrequency. Considerably greater power can then be safely handled by theoscillator, allowing stages of amplification to be dispensed with inmany cases, with a decided advantage from the standpoints of weight,simplicity and economy of first cost and operation.

The invention consists of a vacuum tube oscillator, more especiallydestined for use in radio transmission, in which the proper degree ofreaction, necessary for grid excitation, is provided through thecombined effects of two devices, one

of these utilizing the piezo-electric properties of a crystal, and theother utilizing a common method of reaction. The effect of each deviceis limited by appropriate means and in a measure calculated, on the onehand, to insure sufiicientfrequency stability, and on the other hand toreduce crystal action to within safe margins, even when said crystal iscut for frequencies higher than 7,000 kilocycles (when it is very thinand more subject to damage) and high plate power is applied, for example200 watts.

The diagram of Figure 1 shows one of the possible known circuits inwhich the piezo electric action of a crystal creates a condition of thegrid circuit in which grid excitation and oscillation are possible.

Diagram of Figure 2 shows one of the possible known arrangements where acrystal used to stabilize the frequency of an oscillator, but where acertain degree of reaction and grid excitation are providedindependently of piezo-electric action. If the crystal is removed fromthese circuits, they can still oscillate, without stabilization, or atleast are near the starting point of oscillations.

The diagram of Figure 3 shows a well known form of oscillator in whichpower is fed back from plate to grid through the internal tubecapacities. Condenser C may be emitted if the grid-filament capacitysuffices to give circuit 01 a correct natural frequency. In some casesin fact, this correctnatural frequency may be obtained by placing inseries a capacity (not shown on the figure) in the grid filamentcircuit.

Diagram Figure 4 illustrates, by way of example, one way in which theinvention may be carried out.

The curves in Figure 5 show oscillation frequencies, plotted againstcapacities of plate tuning condenser CV, for different circuitarrangements.

To better explain the present invention, we will recall two of theconditions which any vacuum tube circuit must fulfil in order togenerate oscillations of a given frequency.

Condition A.A coupling of the plate and grid circuits must be providedthrough which the grid potential will receive an adequate impulse forevery oscillation of the plate potential. 1

In certain oscillators this coupling consists only of the internal tubecapacity, in others an additional coupling is provided.

Condition B.-The grid potential must oscillate easily, at the chosenfrequency, relatively to the filament potential. In other words, aresonance effect in the grid circuit must allow grid oscilla ingpotentials to build up, at the frequency of the above mentionedimpulses. depend on the device through which the grid is connected tothe filament.

A complete failing of either of the conditions A or B must necessarilyprevent any grid excitation (and consequently, any generation of continuous oscillations), but a lessening of one of the two may, withincertain limits, be compensated by an increase in the other.

If the oscillator is to work with good output efliciency, a certaindegree of grid excitation must nected between the grid and the filament.

The piezo-electric mechanism depends on certain elastic deformations ofthe crystal, which are alternatively effects or causes of potentialdifferences between its faces. These deformations have a very markedtendency to follow a characteristic fundamental oscillatory rhythm, ofextremely constant frequency, and sometimes other tendencies as Well,but less marked, to follow certain secondary frequencies.

At the fundamental frequency of the crystal (and eventually at thesecondary frequencies also), this resonance phenomenon will bring aboutthe building up of the grid potential oscillations required by conditionB. But, at the other frequencies, even very close to those abovementioned, there will be no resonance response or building up of thesepotentials and therefore no possibility of sustained oscillations.

The universally known circuit shown in Figure 1 illustrates this method.The plate-grid capacity of the tube provides the coupling required bycondition A; it is represented in the diagram as a, and is shownconnected with dotted lines. This coupling carries voltage impulses fromthe plate to the grid, and as the grid-circuit oscillates by thepiezo-electric effects of the crystal (condition B), the grid voltagesbuild up until continuous oscillations are maintained. A satisfactoryoutput efficiency may be obtained in the load resistance R, which may bereplaced by an an tenna or amplifier coupled to the plate coil. The

degree of grid excitation will depend on the difference between thenatural frequencies of the plate circuit and of the crystal, and cantherefore be adjusted by plate condenser CV. It is found thatoscillation can take place for a wide band of adjustments of thiscondenser. The amount of excitation depends on the more or lessdifference between the natural frequencies of the plate and gridcircuits. The frequency is controlled by the crystal and remains veryconstant throughout these adjustments. This is illustrated by the curve1.1 in Figure 5, where oscillation frequencies are plotted againstvalues of plate condenser CV, when these values decrease from maximumtowards minimum. This circuit is subject to the general drawback ofcrystal oscillators, as a plate power of the order of 10 watts is theutmost which may be applied without damaging the crystal.

Method II.Another method of fulfilling condition B is to connect thegrid and the filament to different points of a tuned circuit 01 havingan adequate resonance frequency. An example of this method isillustrated in Figure 3. The grid potential can oscillate easily atthis, and at all other very close frequencies. If the plate circuit istuned closely enough to these, the potential impulses reaching the gridthrough the tube capacity will build up the grid voltages high enoughfor oscillations to be maintained. The amount of grid excitation, as inthe case of Method I, can be adjusted by means of condenser CV, and thetube may oscillate for a wide range of these adjustments. But this time,variations of CV will cause corresponding variations in the oscillationfrequency, as may be seen in curve 2-2 in Figure 5.

The present invention consists in fulfilling condition B by acombination of the devices, characterizing Methods I and II (a crystaland a tuned grid), while providing adequate means for limiting andadjusting the extent to which the resonance effect of each device willaffect the grid excitation. These adjustments must allow the total gridexcitation to reach its optimum value without exceeding it. If then theexcitation by the crystal is increased, the excitation by the tuned gridcircuit must be correspondingly decreased and vice versa.

Figure 2 shows, by way of example, a known vacuum tube oscillatorarrangement, above referred to, comprising a tuned grid circuit and acrystal for stabilizing the frequency when the natural frequency of theplate circuit, tuned by the condenser CV is included within certainlimits. The feed back of said device is such that, if the crystal wereremoved, the circuit might still oscillate or have suificient gridexcitation to almost oscillate.

The method of carrying out the invention illustrated by way of examplein Figure 4 may be looked upon as derived from the circuit of Figure 2to which have been added appropriate devices for adjusting the effectsof crystal resonance and grid circuit resonance to certain preferredratios, allowing the oscillator to handle more power with less strain onthe crystal (by applying equivalent devices to other circuits of thesame kind, other forms of the invention would be obtained).

Grid excitation by the crystal will be adjusted by means of a capacity Q(Figure 4) or suitable resistances (not shown) or an inductive coupling(not shown). Excitation by grid circuit resonance can be cut down andadjusted by one of the following means: 7

(a) By adjusting plate condenser CV, thus bringing the plate tuningfarther from or closer to the grid tuning,

(b) By giving a suitable damping to the tuned grid circuit, by means ofone or more of the auxiliary resistances such as R1, R2, R3, R4, R5, Re.It will not be necessary to employ them all simultaneously. In reality,some of them will always have values close to zero or practicallyinfinite. In selecting them, one will have to take into considerationthe natural resistances of all parts already present in the gridcircuit;

(0) By adjusting the inductance-capacity ratio of the grid circuit.

With the arrangement described, it is found that, for decreasingcapacities of CV, the oscillation frequency varies as shown by curve 3in Figure 5. Up to point P3, curve 3 practically coincides with curve 2.The frequency varies as if no crystal were present. Beyond P3, thestabilizing action of the crystal commences, and curve 3 separates fromcurve 2 and tends to coincide with curve 1 from P1 to P2. At P2,stabilization ceases abruptly, and curve 3 ascends vertically to P4, tocoincide again with curve 2 from there on. A very stable frequency isobtained between points P1 and P2. These points will be brought closertogether if capacity Q is reduced. The distance between them mightcorrespond to differences in plate tuning of the order of 0.1% to 5% infrequency.

For a given degree of total or combined excitation of the grid circuit,it is found:

1That when the ratio excitation by crystal action excitation by tunedgrid circuit is increased:

(a) The frequency is stabilized for an increased range of platecondenser tuning;

(b) R. F. currents flowing through the crystal increase.

2That decreasing this ratio will bring about the reverse phenomena.

In view of these facts, the devices characterizing the invention will beso adjusted as to give a value to said ratio which will obey thefollowing conditions:

l--It must be sufficiently high to allow frequency stabilization to takeplace on a plate tuning range large enough not to be exceeded byaccidental detuning causes (mechanical. movements and vibrations,variations in power supply, modulation, etc.

2It must be low enough for the R. F. currents flowing through thecrystal and voltages on same to be small, allowing the power input andoutput to be increased without danger for the crystal.

In order to judge these values correctly, a radio frequency milliameterA may be connected in series with the crystal. By adjusting capacity Q,one will then broaden the stabilization range P1P2 (Figure 5) as much aspossible without letting the current in A exceed reasonable read ings,indicating crystal safety.

With low power, a high ratio of excitation by crystal action excitationby tuned grid circuit may safely be used, with a broad stabilizationrange.

With increased power, the ratio must be lowered, with consequentnarrowing of the stabilization range.

ExampZe.The following typical results have been given by the circuits ofFigures 1 and 4, using a watt tube (radiotron U. V. 211 of the RadioCorporation of America). The load R was replaced by coupling a zeppelinantenna to the plate coil. The wave length was 42 meters.

In both cases, the same order of stability was maintained and greaterpower has frequently been handled, crystals giving service for severalmonths.

The circuit of Figure 2, which has been chosen :as a basis for themethod, that has been discussed .above, for carrying out the presentinvention, is proposed as a nonlimitative example. By treating othercircuits in an equivalent manner, other forms of the invention would bearrived at.

What I claim is:

1. An electric oscillation generating device in cluding a threeelectrodes vacuum tube, means for applying a suitable potential to theplate, an oscillatory plate circuit, an oscillatory grid circuit, apiezo-electric crystal, means for connecting said crystal in parallelwith the oscillatory grid circuit, means for adjusting the degree of thegrid excitation caused by the crystal and means for adjusting the degreeof grid excitation caused by the oscillatory grid circuit.

2. An electric oscillation generating device including a threeelectrodes vacuum tube, means for applying a suitable potential to theplate, an oscillatory plate circuit, an oscillatory grid circuit, apiezo-electric crystal, means for connecting said crystal in parallelwith the oscillatory grid circuit, an adjustable condenser, means forconnecting said condenser in series with the crystal and means foradjusting the degree of grid excitation caused by the oscillatory gridcircuit.

3. An electric oscillation generating device including a threeelectrodes vacuum tube, means for applying a suitable potential to theplate, an oscillatory plate circuit, an oscillatory grid circuit, apiezo-electric crystal, means for connecting said crystal in parallelwith the oscillatory grid circuit, means for adjusting the degree of thegrid excitation caused by the crystal and adjustable resistancesconnected in the grid oscillatory circuit in order to adjust the degreeof the grid excitation caused by said oscillatory circuit.

EDMONDO RUSPOLI.

